Nanocrystal assemblies, electronic properties

Optical characteristics of nanocrystals and nanocrystal assemblies have been extensively studied in the past,66,67 whereas their electronic properties have attracted considerable attention only recently.68"70 Charge transport in... 

Sumanene (2) and sumanenetrione (120) have similar bowl structures, but different electronic properties [155]. Their nanocrystals can form core-shell assemblies, and aggregates of 120 are wrapped by nanocrystals of 2. The assemblies showed that amplified photoluminescence of sumanenetrione nanocrystals was caused by energy transfer from sumanene nanocrystals through the nanocrystaUine interface. 

Gray DG (1994) Chiral nematic ordering of polysaccharides. Carbohydr Pol5fm 25(4) 277-284 Greiner A, Hou H, Reuning A, Thomas A, Wendorff JH, Zinunermami S (2003) Synthesis and opto electronic properties of cholesteric cellulose esters. Cellulose 10(l) 37-52 Habibi Y, Lucia LA, Rojas OJ (2010) Cellulose nanocrystals chemistry, self-assembly, and applications. Chem Rev 110(6) 3479-3500... 

C. Electron Transport Properties of Nanocrystals Either Isolated or Self-Assembled in 2D and 3D Superlattices... 

The ability to synthesize lattices of nanociystals have led to explorations of their collective physical properties. Thus, it is observed in the case, of Co nanociystals (5.8 nm) that, accompanying lattice formation, the blocking temperature increases. 421 FePt alloy nanocrystals yield ferromagnetic assemblies for which the coerrivity is tunable by controlling the parameters such as Fe Pt ratio and the particle size.1431 The evolution of collective electronic states in CdSe nanocrystals have been followed by optical spectroscopic methods. Compared with isolated nanocrystals, those in the lattice exhibited... 

We have hitherto discussed in the earlier sections, electronic structure and properties, chemical reactivity and self-assembly of nanocrystals, particularly those of metals. Hie discussion should suffice to illustrate how size if a crucial factor in deciding the chemistry in the nano regime. These size dependent properties form the basis of nanoscience, where the properties are exploited for possible applications. 

One-dimensional (ID) nanostructures such as nanowires, nanorods and nanobelts, provide good models to investigate the dependence of electronic transport, optical, mechanical and other properties on size confinement and dimensionality. Nanowires are likely to play a crucial role as interconnects and active components in nanoscale devices. An important aspect of nanowires relates to the assembly of individual atoms into such unique ID nanostructures in a controlled fashion. Excellent chemical methods have been developed for generating zero-dimensional nanostructures (nanocrystals or quantum dots) with controlled sizes and from a wide range of materials (see earlier chapters of this book). The synthesis of nanowires with controlled composition, size, purity and crystallinity, requires a proper understanding of the nucleation and growth processes at the nanometer regime. 

In the second part of this work we review our theoretical and experimental works to obtain enhanced two-photon cross-sections by using the super-linear response of centrosymmetric monomers that are coherently coupled. In this alternative approach, the nonlinear material consists of an assembly of nonsubstituted /r-electron systems that are coupled by dipole-dipole interactions. The monomer two-photon term is a pure transition dipole term ( UQ,jU,2). Typical materials can be molecular aggregates, nanocrystals, oligomers, and dendrimers. The dipole-dipole interactions determine the size dependency of optical properties, and in particular of two-photon cross-sections. 

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